Referring to FIG. 1, a conventional multistep focusing electron gun for a color cathode ray tube comprises a cathode K, a control grid G1, and a screen grid G2 all together constituting a triode section, and also electrodes G3 to G8 constituting auxiliary lenses and a major lens of a main lens system, as shown in FIG. 1. In a conventional multistep focusing electron gun having the above construction, a voltage below 1 KV is supplied to the electrodes G2, G4, and G6, a voltage below 10 KV is supplied to the electrodes G3, G5, and G7, and a voltage below a maximum 30 KV is supplied to the anode, i.e. the electrode G8. At this time, a first focus voltage of a certain potential is supplied to the electrodes G3, G5, and G7, and a second focus voltage lower than the first focus voltage is supplied to the electrodes G2, G4 and G6. According to the voltage supplying method, a first unipotential static lens is formed by the electrodes G3, G4, and G5, a second unipotential static lens is formed by the electrodes G5, G6, and G7, and a bipotential static lens is formed by the electrodes G7 and G8.
Referring to FIG. 2, in this same conventional multistep focusing electron gun for a cathode ray tube, after thermal electrons emitted from the cathode K are formed into an electron beam by the electrodes G1 and G2, the beam is preliminarily accelerated through the first unipotential static lens and the second unipotential static lens, and is finally focused and accelerated by the bipotential static lens. At this time, the electron beam is gradually diverged while passing the first and second unipotential static lenses, in which the diverging angle .theta.2 of the electron beam in the second unipotential static lens is larger than the diverging angle .theta.1 in the first unipotential static lens.
The reason for such gradual divergence is that an electron beam passing through hole H of the electrode G6 among the electrodes G5 to G7 which constitute the second unipotential static lens, has a diameter equal to those of electron beam-passing holes of the electrodes G5 and G7 respectively disposed at the front and at the rear of the electrode G6, and the thickness T of the electrode G6 is relatively thick.
Accordingly, this conventional electron gun cannot provide a good focus characteristic. In this electron gun, to form an electron beam having a good focus characteristic, the diverging angle of the second unipotential static lens should be reduced. To reduce the diverging angle, the thickness T of the electrode G6 should be reduced or the electron beam-passing hole H of the electrode G6 should have a diameter larger than those of the electron beam-passing holes of the adjacent electrodes G5 and G7 disposed respectively at the front and the rear of the electrode G6. However, there is a limitation in reducing the thickness of an electrode because a thin thickness T of the electrode G6 deteriorates its mechanical strength, thereby causing the electrode G6 to be subject to deformation by a compressive force applied when all of the electrodes are fixed to supporting beads. When the electron beam-passing hole H of the electrode G6 is formed so as to be larger than those of the electrodes which are respectively at the front and at the rear of the electrode G6, the positions can not be exactly set by the guide rod for setting the relative position to be inserted to the electron beam-passing hole while the electrodes are being assembled into one structure, thereby deteriorating the degree of precision with which the electrodes are assembled to form an electron gun.